The present invention generally relates to drilling bits utilized in the oil well drilling industry and in the mining arts, and more particularly involves a unique metallic composition for the cutting elements utilized in drilling bits. In the conventional drill bit technology, there are generally two kinds of rolling cutter drill bits, as well as what is termed drag bits having no rolling elements. The rolling cutter drill bits are generally of the type having cantilevered frusto-conical cutters such as the tri-cone bit, and there are additionally bits having cutters mounted transversely on axles supported at each end by saddles, which in turn are affixed to large cutting heads. This second type of rolling cutter bit primarily is used in the mining and tunneling industries. In the tri-cone rolling cutter type of bit, there are generally two kinds of cutter structures utilized, the "milled tooth" cutter, and the insert cutter. In the milled tooth cutter, a large forging is milled away, leaving protruding, sharp, wide chisel-shaped teeth as the cutting elements. These projecting teeth may have a hard material, such as tungsten carbide, welded to their faces to increase their erosion resistance. The cutter bodies themselves may be carburized and hardened to increase their resistance to breakage and wear.
In addition to the milled tooth cutters, rolling cutter drill bits commonly utilize insert type cutters wherein a smaller original cutter body is utilized with a minimum amount of machining, and holes are drilled circumferentially around the cutter body to receive hard metal cutting inserts which are pressed thereinto. These hard metal inserts generally are formed of a tungsten carbide composite made in a generally cylindrical shape with a pointed protruding portion. The insert type cutter bodies generally are carburized and hardened prior to insertion of the inserts.
In the mining industry, the saddle type cutters most often used are the milled tooth variety, although the insert type cutters are becoming more widely used. The formation of these cutters is similar to that as described above with respect to the tri-cone drilling bit cutters. In the formation of the rolling cone cutting structures utilized both in the tri-cone bits and the mining bits, the two types of cutters can generally be classified as utilizing both gradient techniques and composite techniques, although none of the conventional cutters have combined these two techniques to arrive at a gradient composite metallic structure.
The term "composite" is used in the microstructural sense herein, as is commonly known in the fields of metallurgy and materials science. It refers to materials having microstructures composed of at least two individual phases, the volume fraction of a minor phase being at least 10%, which are bonded together in such a manner that the average properties of the composite are determined by the individual properties and morphology of each phase. This usage is distinct from that where an engineering structure may be referred to as a "composite" when it is comprised of two or more distinct, relatively large regions, bonded, coated, welded, or otherwise joined or internally transformed, forming a heterogeneous unit. See--THE PRINCIPLES OF ENGINEERING MATERIALS, Barrett, Nix and Tetelman, Prentice-Hall, Inc., 1973, pp. 316-317.
For example, both the milled tooth cutter and the insert type cutter utilize the composite structures in that they both have a steel alloy cutter body to which is added a hard metal cutting surface, or cutting element. In the milled tooth cutter the composite hard metal element is added as a tungsten carbide alloy weldment which is fused to the cutting surfaces on the teeth, the gage, and portions of the cutter body. In the insert type cutter, the composite element is added by the insertion of the cemented carbide insert into the alloy steel cutter shell. The result of these two types of composite metallurgical construction is a "metallurgical notch", where a very sharp gradient is formed across the interface between the hard metal and the alloy steel. In addition to this metallurgical notch, or discontinuity, the composite formed thereby also suffers from a disadvantage in that a geometrical notch is also usually formed at the juncture. These metallurgical and geometrical notches serve to weaken the resulting composite metal component and contribute to earlier failure of the cutting structure. These discontinuities in elastic moduli, coefficients of thermal expansion, and yield characteristics limit drilling performance by affecting the residual stress distributions and applied stress distributions in service. These characteristics and changes result from all of the different techniques which have been utilized in conventional cutter construction for reducing deformation and improving wear-resistant qualities on drilling equipment.
The composites utilized in conventional cutters have increased the mechanical strength, toughness and hardness but have not efficiently optimized these characteristics for drilling equipment. In addition to the welding of hard metal, such as cemented carbides, on the cutting structures, other conventional techniques have involved brazing of the cemented carbides, plasma spraying of cemented carbide coatings, and chemical and electrical deposition of coatings having high carbide fractions. All of these techniques suffer from the above-mentioned mechanical and metallurgical discontinuities at the joint interface. Likewise, the insert cutter construction has been utilized to improve the mechanical strength, toughness and wear resistance of the cutting structure, but it still suffers from the elastic strain requirements of the interference fits, in addition to the limitations of the steel-composite interface on load bearing ability.
The use of mechanical property gradients in conventional drilling tools has been known and accepted for many years. For example, gradients are introduced into the cutting structures by the case hardening, carburizing treatment of steels. The resultant gradient of a carburized case-hardened steel comprises a hard brittle outer surface shell with a tapering-off of the hardness and increase in toughness towards the interior of the part. This has been successful in reducing galling and spalling of bearing surfaces and other high unit loading contact areas, but offers little improvement to erosion resistance which is prevalent in rock drilling. Also, this type of gradient is generally relatively shallow, usually extending no more than 0.050 inches into the steel component, thus subjecting the surface to cracking or failure by plastic deformation. Other types of mechanical property gradient-producing processes include laser and induction hardening, nitriding and boronizing.
The present invention overcomes these disadvantages and provides an optimum cutting structure by the use of gradual or continuous gradients across the geometry of the cutting structure. This continuous or gradual gradient substantially eliminates the interface and the resultant geometrical and metallurgical notches found in the conventional cutter construction. The elimination of the discontinuities may involve varying several different parameters to achieve different desirable techniques. For instance, the composition, the fraction, the shape, the size and the distribution of phases in a cemented carbide composite may be systematically varied by powder metallurgy techniques to produce an insert with continuously varying properties. The gradient through the insert can be arranged so that a hard, stiff, abrasion-resistant cemented carbide structure exists at the tip of the insert, merging into a tougher, softer cemented carbide structure in the regions of high bending stress lower in the insert body. The gradient across the inserts can also be arranged such that when fused to the normal alloy steel cutter shell, the attachment surface of the insert can be substantially of the same composition as that of the alloy steel cutter shell so that the added insert becomes an integral part of the cutting structure as though originally formed therewith, and a hard metal core extends downwardly along the central longitudinal axis of the insert.
In a second embodiment of the invention, the cutting structure is formed in a single operation rather than by the addition of inserts to a cutter shell. In this embodiment, the cutter and the teeth structure are formed in a single manufacturing operation utilizing powder metallurgy techniques. A programmable mixing system for mixing the alloying components of a powdered metal alloy serves to place the proper concentrates of the cemented carbides in the locations requiring the properties of cemented carbides and gradually reducing the cemented carbide fraction as you move geometrically away from these critical points. The resulting cutting structure therefore has concentrated fractions of cemented carbide in the high-stress, high-erosion areas with a gradual decrease in the hard metal component away from these critical areas towards the body of the cutter. The alloyed powder metallurgy components are then densified into a single integral cutting structure utilizing conventional powder metallurgy techniques, such as hot isostatic pressing. Then the completed cutter is removed from the pressing die and minor machining operations can be performed to create smooth bearing surfaces and seal surfaces within the cutter where required. Thus, it can be seen that the resulting drilling bit cutter offers an optimum metallurgical cutting structure in that it utilizes the desirable effects of the composites, such as cemented carbides, in the locations on the cutter where such characteristics are desirable, and the desirable characteristics of a tough resilient core, such as the alloy steels, for strength and foundation in the cutter shell itself with a smooth continuous gradient between the cemented carbide and the alloy steel to greatly reduce or eliminate discontinuities and their resultant stress risers. In addition, the locations of the gradients and the gradient rates can be manipulated to provide favorable compressive residual stress patterns in a finished component, thereby raising the effective fracture resistance of the resulting cutting structure.